In view of the mountainous city's road traffic planning needing to adapt more to the mountainous environment's topographic and geomorphological characteristics, traffic demand and traffic organization, this paper,which integrates the thinking mode of adaptability theory, takes the research of road traffic planning of mountainous city as the core content and puts forward the design method of mountainous city planning. Combining the coupling, collaboration and organizational thinking of adaptability theory, this method explores the planning method of road traffic system in accordance with the topographic characteristics and construction laws of mountainous cities from three aspects: road network system collaboration macro-topography, pedestrian space coupling micro-topography, and characteristic traffic organization connection bottleneck. Relevant cases prove that the road network planning guided by this planning method is helpful to form the urban morphological characteristics and spatial environment with mountain characteristics. This method is in line with the topographic characteristics of mountainous cities and road traffic demand planning method. It can improve the relevant design theory and guide the construction practice of mountainous cities.